Fluidized powder flowrate measurement method and device

ABSTRACT

The flowrate of powder in an air-powder mixture circuit comprising a mixture drive area defined by a powder drive device comprising air injector means adapted to inject air in the direction in which the mixture is to be driven is measured by measuring the pressure difference across the drive area and the injected air flowrate and computing the powder flowrate as a function of these two variables.

This is a division of application Ser. No. 07/925,638, filed on Aug. 7,1992, now U.S. Pat. No. 5,351,520.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a method of measuring the flowrate of fluidizedpowder in a circuit including an air-powder mixture drive area intowhich air is injected.

The invention enables evaluation of the powder mass flowrate on thebasis of measurements at selected points of a mixture drive devicedefining said drive area. It also enables flowrate regulation, reportingof absence of powder and determination of the degree of wear of anessential component of the drive device which is also part of the powderflowrate measuring means.

The invention also concerns a powder flowrate measuring deviceimplementing the above method.

2. Description of the Prior Art

U.S. Pat. No. 4,480,947 describes a powder flowrate measuring devicedisposed between a fluidized powder storage tank and a powder conveyordevice forming a sort of compressed air pump whose function is to impartto the air-powder mixture sufficient energy to enable it to be conveyedin a pipe to a station where it is used. The measurement principleemployed is to inject air at a constant flowrate into a pipe section ofpredetermined length and cross-section and to measure the pressure dropacross this pipe section. The only function of the air injected is tomove the powder across the orifice at a predetermined speed. In otherwords, a head loss is generated only to measure the mass flowrate ofpowder. Generating this head loss disturbs the operation of the powderconveyor device further downstream. Also, the accuracy of themeasurement is reduced over a period of time because of wear of the pipesection in which the head loss is produced. The degree of wear becomesunacceptable after a varying period of time dependent on the operatingconditions. The device described comprises no means of evaluating thiswear. Finally, the construction of the known system is highly complexbecause it requires a controlled flowrate compressed air supply and adedicated arrangement for generating and measuring the head loss.

The invention proposes to use another type of measurement which is freeof the drawbacks mentioned above and employs only a limited number ofsensors in a particularly simple arrangement.

SUMMARY OF THE INVENTION

In one aspect, the present invention consists in a method of measuringthe flowrate of powder in an air-powder mixture circuit comprising amixture drive area defined by a powder drive device comprising airinjector means adapted to inject air in the direction in which saidmixture is to be driven, which method measures the pressure differenceacross said drive area, measures the injected air flowrate and computessaid powder flowrate as a function of these two variables.

The flowrate of the injected air can of course be deduced from thepressure measured on the upstream side of the injector.

In other words, the invention consists in measuring the operatingparameters of the powder drive device conventionally used in manyinstallations including an air-powder mixture circuit. One such powderdrive device comprises a throat usually called a "Venturi" and an airinjector disposed axially at the throat entry and connected to acompressed air supply. The effect of the injected air is to give to thepowder its momentum or impulse. The above two measurements amount tomeasuring momentum of the air and of the powder before mixing and aftermixing, during passage through the throat. The powder mass flowrate isdeduced from these two measurements. The throat is a tube of smalldiameter serving as the conveyor over the required pipe length and ofsufficient length (5 to 15 times its diameter, for example) to achievehomogeneous mixing of air and powder. It preferably has a divergentoutlet to minimize the head loss. It may comprise a convergent inlet.The powder is entrained by the air injected at high speed in the throat.Note that in this type of arrangement the powder mass flowrate isinversely proportional to the pressure difference across said drivearea. On the other hand, in the prior art system described above thepower mass flowrate is virtually proportional to the head loss measuredacross the pipe section. The measurement may seem relatively simple butthe equipment is in fact very complicated.

In a second aspect, the invention consists in a device for measuring theflowrate of powder in an air-powder mixture circuit comprising drive airinjector means operative in a drive area of said circuit, pressuremeasuring means connected to either side of said drive area, airflowrate measuring means connected to said air injector means andcomputing means adapted to receive signals produced by said measuringmeans and to produce an information signal representative of the powderflowrate.

The injected air flowrate measuring means may comprise a pressure sensorconnected to the inlet side of an injector.

In this way two sensors are sufficient to obtaining the data needed toevaluate the powder mass flowrate in the air-powder mixture crossing thedrive area. The pressure measuring means comprise a differentialpressure sensor connected to each side of the drive area and the airflowrate measuring means may comprise a pressure sensor adapted todeliver a signal representing the total head pressure of the injectedair. The sensors may be of any known type, including piezoelectric orstrain gauge sensors.

The invention will be better understood and its other advantages willemerge more clearly from the following description given by way ofexample only with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing an installation for electrostatic sprayingof thermofusible powder paint incorporating powder mass flowratemeasuring means in accordance with the invention.

FIG. 2 is a graph showing the relationship between the powder flowrateand the variables measured in the powder flowrate measuring deviceincorporated in the installation from FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an installation for electrostatic spraying ofthermofusible powder paint comprises a powder storage tank 11, a powderpaint sprayer 12 and an air-powder mixture circuit 13 extending from thetank 11 to the sprayer 12. The latter includes an electrode 15 connectedto a high-tension voltage supply 16. The storage tank 11 conventionallyhas a double bottom incorporating a porous wall 18 through which air isinjected to fluidize the powder in the tank. The circuit 13 includes asuction tube 20 descending vertically into the tank. A powder drivedevice 22 known in itself is inserted into the circuit 13 at the top ofthe suction tube 20. It comprises a throat 24 usually called a "Venturi"and air injector means 25 adapted to inject a high speed air jet intothe throat 24 in the axial direction (i.e. in the required propagationdirection for the air-powder mixture). The injector means comprise aninjector nozzle 26 directed axially relative to the throat and acompressed air supply 28 feeding said injector through a valve 30. Thisarrangement creates a drive area which in this example extendssubstantially from the top of the pipe 20 to the outlet of the throat24. The throat 24 is advantageously a removable part of the circuit 13which can be replaced easily.

The flowrate measuring device includes at least two pressure measuringmeans 32, 34. In this example a first pressure measuring means comprisesa differential pressure sensor 32 connected to either side of the drivearea to measure directly the pressure difference Δp between its inletand its outlet. A second pressure measuring means comprises a sensor 34for measuring pressure relative to atmospheric pressure or gagepressure, connected to the air injector means 25 on the inlet side ofthe injector nozzle 26. In other words, it is connected to a pipesection 33 connecting the compressed air supply to the nozzle 26. Theflow cross-section in the pipe section 33 is very large as compared withthat of the injector nozzle 26. The speed at which the air flows at thepoint to which the sensor 34 is connected may be regarded as virtuallyzero. The sensor 34 therefore outputs a signal representing the totalhead pressure of the air expelled through the nozzle 26. Also,variations of atmospheric pressure can be neglected (or even corrected)and the sensor 34 therefore regarded as producing a signal representingthe absolute total head pressure Pi of the drive air.

It has been found that the mass flowrate Dp of powder across the drivearea and therefore the flowrate of the powder in the circuit 13 can bededuced from the measured values of Δp and Pi respectively supplied bythe sensors 32 and 34.

To be more precise, it has been found that the flowrate Dp can bedetermined with sufficient accuracy from the following equation:##EQU1## where K₁, K₂ and K₃ are positive constants and Pat representsthe atmospheric pressure which is regarded as constant in the termincluding it. It has been found that the head loss Δh in the throat isdependent virtually only on the pressure Pi. As previously mentioned,this pressure is representative of the injected air flowrate.

Consequently, it is seen that the powder mass flowrate can be determinedeasily by a computer for which the input data is the signals produced bythe two sensors 32 and 34. In the FIG. 1 installation a computer 36 isprogrammed to perform cyclical computation, continuously, of the powderflowrate from the signals produced by the sensors 32 and 34. Thiscomputer drives display means 38 indicating the instantaneous value ofthe flowrate. The computer 36 is also part of a powder flowrateregulator loop 40 controlling the valve 30. This is a proportional valveand its control input is connected to the output of a comparator 41. Thecomputer 36 has a data output 42 supplying a signal representing thepowder mass flowrate and used as an error signal applied to an input 43of the comparator 41. The other input 44 of the comparator 41 isconnected to the output of a set point generator 45 enabling the powderflowrate to be set to a chosen value.

Finally, the installation comprises another relative pressure sensor 48connected on the input side of the throat 24, to be more precise to theupper part of the suction tube 20. This sensor measures the pressure atthis point which varies significantly when the storage tank 11 no longercontains any powder. The computer 36 is programmed to detect thispressure variation so that an alarm system can be actuated or sprayinghalted.

It is clear from the above description that the installation shown inFIG. 1 enables not only continuous monitoring of the powder flowrate bymeans of the display device 30 but also keeping the flowrate at apredetermined constant value as the valve 30 is controlled by aregulator signal derived from the signals supplied by the sensors 32 and34.

FIG. 2 is a calibration graph in which the powder flowrate Dp (in gramsper minute) is plotted on the vertical axis as a function of thepressure difference Δp in millibar plotted on the horizontal axis. Thiscalibration graph comprises a family of curves each representing a giveninjection pressure Pi, the variation of Pi from one curve to the nextbeing 0.5 bar. How to "program" this calibration graph into the computer6 will be obvious to the man skilled in the art.

According to another advantageous feature of the invention the computer36 is also programmed to integrate with respect to time during anyperiod in which the installation operates at least the two valuesmeasured by the sensors 32 and 34 or one of these values and thecomputed powder flowrate and to deduce therefrom an indication of thedegree of wear of said throat as a function of these two integratedvalues. It is found that the wear of the throat 24 is a function of thekinetic energy of the air-powder mixture flowing through it. The densityof the air-powder mixture is a function of the powder mass flowrate andthe air flowrate while the air-powder mixture speed is a function of thedrive air flowrate. Consequently, the degree of wear is a function ofthe powder flowrate and of the air flowrate. The air flowrate is deducedfrom the measured parameter Pi supplied by the sensor 34. The powderflowrate is a function of this same parameter Pi and the pressuredifference Δp measured by the sensor 32.

There is claimed:
 1. In an air-powder mixture conveying installation comprisingi) a suction tube connected to a fluidized powder reservoir, ii) air-powder mixture drive means connected to said suction tube, said air-powder mixture drive means including throat means having an inlet and an outlet and an air injector means comprising an air nozzle, said nozzle being directed axially relative to said throat means and facing said inlet thereof and said nozzle being positioned relative to said suction tube and said throat means for effecting mixing of air from said nozzle and powder from said reservoir in said throat means to create an air-powder mixture, and iii) a controllable compressed air supply connected to feed said air nozzle, a powder flowrate measuring arrangement comprising:pressure difference measuring means disposed for measuring a pressure difference between said inlet and said outlet of said throat means of said drive means air flowrate measuring means for measuring flowrate of air ejected from said air nozzle of said drive means, and computing means, connected to said pressure difference measuring means and said air flowrate measuring means, for computing the flowrate of powder as a function of the measured difference and air flowrate.
 2. The powder flowrate measuring arrangement of claim 1 wherein said throat is a Venturi device.
 3. The powder flowrate measuring arrangement of claim 1 wherein said air flowrate measuring means comprise a pressure sensor connected to said air injector means upstream of said nozzle.
 4. The powder flowrate measuring arrangement of claim 1 wherein said throat means and said air injector means are arranged at an upper end of a pipe forming said suction tube, arranged to extend downwardly into said fluidized powder reservoir.
 5. The powder flowrate measuring arrangement of claim 1 further comprising a pressure sensor disposed at said inlet of said throat means to supply a signal representative of the presence of powder in said drive means.
 6. The powder flowrate measuring arrangement of claim 1 wherein said computing means are programmed to compute continuously the powder flowrate from signals supplied by said pressure difference measuring means and said air flowrate measuring means.
 7. The powder flowrate measuring arrangement of claim 1 wherein said air-powder mixture drive means define a flow path between said air nozzle and said outlet end of said throat means and said throat means has a narrowed portion with a smaller diameter than any point along said flow path between said narrowed portion and said air nozzle.
 8. The powder flowrate measuring means of claim 1 wherein the flowrate of powder computed by said computing means has an inverse proportional relationship to the pressure difference measured by said pressure difference measuring means. 